A compression ratio of an internal combustion engine is defined as a ratio of a cylinder volume when a piston within the cylinder is at bottom-dead-center (BDC) to the cylinder volume when the piston is at top-dead-center (TDC). In general, the higher the compression ratio, the higher the thermal efficiency of the internal combustion engine. This in turn results in improved fuel economy and a higher ratio of output energy versus input energy of the engine. In conventional engines, the compression ratio is fixed, and thus, the engine efficiency cannot be optimized during different operating conditions in order to improve fuel economy and engine power performance. However, in variable compression ratio (VCR) engines, the engine may be equipped with various mechanisms to mechanically alter the volumetric ratio between the piston TDC and BDC, allowing the compression ratio to be varied as engine operating conditions change. As a non-limiting example, the VCR engine may be configured with a mechanical piston displacement changing mechanism (e.g., an eccentric) that moves the piston closer to or further from the cylinder head, thereby changing the size of the combustion chambers. Still other engines may mechanically alter a cylinder head volume.
VCR engines may be fueled, at least in part, through direct injection (hereafter also referred to as “DI”), wherein fuel is injected directly into the engine cylinders. The timing of the direct injection may be selected as a function of crankshaft position and may be scheduled for a duration. Further, direct injections may be scheduled during an intake stroke, a compression stroke, or a combination of both, called split injection. In conventional engines, a crankshaft position corresponds to a specific position of the piston relative to the cylinder head. However in a VCR engine, since the position of the piston relative to the cylinder head may change based on the compression ratio, the actual injection timing may be different than intended if selected based on the crankshaft position. For example, during the intake stroke while the piston is moving down, the piston will be higher in the cylinder bore at a higher compression ratio relative to a lower compression ratio, and thus, direct injection at the same crankshaft position would result in different actual injection timings for the two compression ratios. If the piston is close to the fuel injector when fuel injection occurs, more fuel may adhere to the piston, resulting in increased particulate matter emissions.
One example approach for adjusting fuel direct injection based on compression ratio in a VCR engine is shown by Kurashima et al. in U.S. Pat. No. 9,291,108. Therein, fuel is injected as either an intake stroke direct injection or a compression stroke direct injection. As a compression ratio increases, intake stroke injection timing is retarded when intake stroke injection is applied. Else, if compression stroke injection is applied, compression stroke injection timing is advanced as the compression ratio increases. In addition, the compression stroke injection is split into a greater number of split injections as the compression ratio increases. As a result, the injection period for each injection is shortened, reducing the penetration of the fuel spray, and thereby the emissions.
However, the inventors herein have recognized that still further improvements in combustion performance can be achieved by leveraging the different evaporation properties of an injected fuel when operating the engine at different compression ratios. For example, fuel may evaporate more easily in the compression stroke, requiring less fuel mass to be delivered. In addition, combustion stability is improved at higher compression ratios. The different fuel evaporation properties can be further leveraged with the different charge cooling effects realized from injecting fuel in the intake stroke relative to the compression stroke to reduce the propensity for abnormal combustion events, as can occur due to extended engine operation in a higher compression ratio. Further still, at higher compression ratios, less compression stroke injection may be used because a smaller cylinder volume at TDC relative to a lower compression ratio creates a rich enough air-fuel ratio in the vicinity of a spark plug to facilitate combustion.
In one example, performance of a VCR engine may be improved by a method for an engine, comprising: actuating a variable compression ratio mechanism of a cylinder to provide a compression ratio selected based on engine operating conditions; and adjusting an amount of fuel direct injected into the cylinder in an intake stroke relative to a compression stroke based on the selected compression ratio. In this way, fuel may be delivered as a split injection over an intake stroke and a compression stroke, with the split ratio adjusted for a given compression ratio, thereby improving fuel evaporation and combustion stability.
As one example, a compression ratio of engine operation as well as a total fuel mass to be delivered may be selected based on engine operating conditions, including engine speed-load and operator torque demand. Based on the selected compression ratio, a variable compression ratio mechanism may be actuated to vary a piston clearance volume. In addition, the total fuel mass may be delivered as a split direct injection with a first portion of the total fuel mass delivered in the intake stroke and a second, remaining portion of the total fuel mass delivered in the compression stroke. As the compression ratio increases, the split ratio of the first intake stroke portion to the second compression stroke portion may be varied so that a larger portion of the total fuel mass is delivered in the intake stroke. Herein, the fuel mass delivered in the compression stroke may be decreased to take advantage of the improved evaporation of fuel injected in the compression stroke at higher compression ratios. At the same time, by delivering a larger fuel mass in the intake stroke when operating at the higher compression ratio, the increased charge cooling effect of the intake stroke injection can be leveraged to reduce the propensity for abnormal combustion events, such as knock and pre-ignition, when operating at the higher compression ratio. Further, multiple short duration intake stroke injections may be used to decrease fuel penetration, especially when tumble is highest, thereby avoiding fuel impingement on cylinder walls and the top of the piston and, in turn, reducing soot formation. The split ratio may be further adjusted based on an alcohol content or octane rating of the injected fuel, such as by further reducing the portion of fuel delivered in the compression stroke as the alcohol content of the fuel increases. In addition to adjusting the fuel split ratio, a timing of the injections may also be adjusted, such as by retarding the start of timing of the intake stroke injection while advancing the start of timing of the compression stroke injection as the compression ratio increases. Further, each of the intake stroke and compression stroke injection may be split into multiple injections.
In this way, a fuel injection profile, including fuel split ratio, injection timing, and injection number, may be adjusted based on a selected compression ratio to improve engine performance. The technical effect of reducing fuel mass injected in a compression stroke while correspondingly increasing fuel mass injected in an intake stroke as the compression ratio increases is that the increased evaporation of fuel in a compression stroke at higher compression ratios may be leveraged to improve combustion stability. Concurrently, the higher charge cooling effect of the intake stroke fuel injection may be leveraged to reduce knock and pre-ignition incidence. By reducing the propensity for abnormal combustion, fuel economy is improved due to reduced need for spark retard or cylinder enrichment, and engine component life is extended. Further, the engine may be operated with a more fuel efficient compression ratio for a longer duration of time. By adjusting the injection timing based on the compression ratio, while taking into account the nature of the VCR mechanism providing the selected compression ratio, fuel may be injected at an optimum piston height within a cylinder, reducing fuel adherence to the piston and, in turn, particulate matter emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.